Field of the Invention
The present invention relates to a step motor driving apparatus employed in a still camera or the like. More particularly, the present invention relates to technique of detecting and securing an initial position or home position of the step motor.
2. Description of the Prior Art
Conventionally, there has been widely used a step motor driving apparatus employing a step motor as a drive source. For example, the step motor driving apparatus is used for feeding a lens system assembled in a still camera or the like. In general, the step motor driving apparatus has an operating member connected to the step motor so as to operate the lens or the like, a link member for connecting the operating member to the step motor, a detecting section for detecting an initial position of the operating member, and a control section composed of a microprocessor for performing an operation control of the operating member in response to a rotation of the step motor. When feeding out the lens, at first, the control section forward rotates the step motor so as to start the operating member. Accordingly, the operating member is moved to a target position from a mechanically stopped extreme position through the initial position or home position which provides a basis or reference for operation. During the course of forward rotating the step motor, it is judged by the detecting section whether or not the operating member performs a predetermined normal movement at a time of passing through the initial position, and whether or not the operating member is moved to the target position after the judgement.
In the conventional step motor driving apparatus mentioned above, the structure is designed such that the operating member passes through the initial position in the middle of the forward rotation of the step motor, and this operation is detected by the detecting section. On the contrary, in a returning operation, the structure is designed such that the operating member passes through the initial position during the course of reverse rotating the step motor, and this returning operation is detected. Therefore, the direction of operation for detecting the initial position is different between the starting time and the returning time of the operating member. Thus, there is generated a backlash, a play or the like in the case that the link member for connecting the step motor to the operating member is constituted by a gear train or the like. Consequently, there is a problem that a lag is generated at a timing for detecting the initial position between the forward rotating time and the reverse rotating time. When the timing lag mentioned above is generated, there is a risk that a system constituted by the step motor as the drive source, the operating member for driving the lens or the like and the control section or the microprocessor is erroneously operated to create a system error.
In order to solve the problems in the prior art mentioned above, the following means is considered. Namely, an inventive apparatus comprises a step motor capable of forward rotating and reverse rotating as a drive source, an operating member connected to the step motor for operation from an initial position to a target position, a link member for connecting the operating member to the step motor, a detecting section for detecting a signal when the operating member comes into a detection zone, and a control section for controlling the operation of the operating member by rotating the step motor. In the apparatus, the control section sets the operating member to the initial position from a mechanically stopped position by forward rotating the step motor at a predetermined step number. At a time of starting the operation of the operating member from the initial position, the control section temporarily reverse rotates the step motor to move the operating member to the detection zone so that the detecting section may detect the signal when the operating member performs a normal movement. Then, the control section responds to the signal to forward rotate the step motor so as to move the operating member to the target position. At a time of a returning operation to the initial position from the target position, the control section reverse rotates the step motor so as to return the operating member from the target position to the detection zone so that the detecting section may detect the signal when the operating member performs a normal movement. Then, the control section responds to the signal to forward rotate the step motor to move the operating member to the initial position, thereby finishing a series of the operations.
Preferably, the control section controls the rotation of the step motor so as to forcibly move the operating member to the initial position when the detecting section fails to detect the signal since the operating member does not perform the normal movement. Further, the control section generates an alarm indicating that an erroneous operation occurs when the control section fails to forcibly return the operating member to the initial position.
In accordance with the present invention, the initial position of the operating member is set to a home position obtained by moving the operating member from the mechanically restricted extreme position to a direction of forward rotation of the step motor at a degree corresponding to some steps. Then, for the initial position detection at a time of starting the operating member, the step motor is temporarily reverse rotated so as to detect the signal during the process. Further, in returning to the initial position after the operation is completed, the step motor is reverse rotated and the initial position of the operating member is detected during the process. Accordingly, as well as an initial position detection at a time of starting and another initial position detection at a time of returning after the operation is completed, the initial position detection is executed by the operation in the reverse rotational direction of the step motor for mechanical resetting purpose, so that it is possible to restrict a timing lag for detecting the signal caused by a backlash or a play of the link member employing the gear train or the like.